This invention refers to a control circuit for a brushless direct current motor and, more specifically, to an electronic circuit for detecting the rotor position by induction voltages.
Brushless direct current motors are of interest in applications requiring reliability and high efficiency such as household appliances, pumps and fans.
Said motors are substantially formed of a stator having coils, a permanent magnet rotor, an inverter feeding current to the stator coils, a position sensor which gives information on the time and period each coil assembly has to be kept powered and a central control which processes current, speed and position of the rotor information and send command pulses to the inverter.
In a circuit control for such motors, the current control during the energization period of each coil assembly is obtained by pulse width modulation of the voltage at a frequency higher than the fundamental wave of the voltage induced to the coil by the moving rotor.
The rotor position detection by inducted voltages is obtained by attenuating the high frequency components coming from the voltage modulation on the coils and then comparing the resulting signals among themselves or comparing each of them with the average of same, thus generating logic signals corresponding to the rotor position.
This rotor position detection technique is mentioned in Japanese document Kokai No. 5280415 and Kokai No. 555035 (A), which employ passive low pass filters of first and second orders RC type to attenuate the high frequency components coming from the voltage modulation on the coils. Such filters, although attenuating satisfactorily the high frequency components, have the drawback of introducing phase displacements to the filtered signals, which phase displacements vary with the frequency and therefore with the rotor speed.
The phase displacement of said filtered signals causes, in turn, the phase displacement of logic signals corresponding to the rotor position in relation to the effective position thereof, feeding therefore, displaced current to the coils in relation to the induced voltage in the same coils. This phase displacement between the fed currents and the induced voltages degrades the motor performance because said motor is not operating at the maximum efficiency condition.
A further control circuit is proposed by Japanese document Kokai 51150624. Such document shows a motor being driven by a cycle converter provided with thyristors wherein the control of the current feeding angle on the coils is efficiently made in a wide speed band by means of a circuit assembly. Said circuit although controlling efficiently the current feeding angle on the coils has the drawback of being complex and is high in cost due to several components of which it is made.